In the PCT application PCT/IL2006/001131, published as WO2007/036937 for “Directional Light Transmitter and Receiver” and in the PCT application PCT/IL2009/000010, published as WO/2009/008399 for “Wireless Laser Power” there are shown wireless power delivery systems based on distributed laser resonators. This term is used in the current disclosure to describe a laser having its cavity mirrors separated in free space, and without any specific predefined spatial relationship between the cavity mirrors, such that the laser is capable of operating between randomly positioned end reflectors. In the above mentioned applications, one use of such distributed laser resonators is in transmitting optical power from a centrally disposed transmitter to mobile receivers positioned remotely from the transmitter, with the end mirrors being positioned within the transmitter and receiver. Such distributed laser resonators use, as the end mirrors of the cavity, simple retro reflectors, such as corner cubes, and cats-eyes and arrays thereof. Retroreflectors differ from plane mirror reflectors in that they have a non-infinitesimal field of view. An electromagnetic wave front incident on a retroreflector within its field of view is reflected back along a direction parallel to but opposite in direction from the wave's source. The reflection takes place even if the angle of incidence of such a wave on the retroreflector has a value different from zero. This is unlike a plane mirror reflector, which reflects back along the incident path only if the mirror is exactly perpendicular to the wave front, having a zero angle of incidence.
Many of such generally available retroreflectors, 15, such as that shown in FIG. 1, generate an optical image inversion around an inversion point 10 situated in the retroreflector (or around points in the case of an array of retroreflectors), or in close proximity thereto, with the reflected beam 11 traversing a spatially different path to that of the incident beam 12, as is shown in FIG. 1.
This inversion around a point causes a number of problems in practical systems:    a. In many such simple retro reflectors, the inversion point is situated in an optically opaque location, where optical access cannot be provided, such as in a corner cube retroreflector.    b. As will be further expounded in paragraphs (c) to (f) below, a distributed laser system designed for practical use should require the placing of optical elements within the cavity. However, this may be problematic, since, following paragraph (a) above, the inversion point in an optically opaque location results in two beams which do not overlap. The explanation for this is that a retro reflector does inversion around the point of inversion 10 in the beam's direction. Thus, expressing the beam directions in in cylindrical coordinates, Theta, the orientation angle, remains constant, R becomes minus R and the direction is reversed. For the two beams to overlap R must equal minus R which dictates that R equals 0, meaning that the reflection must take place at the opaque inversion point. As a result of this lack of overlap, as shown in FIG. 1, placing a required optical element with at least one non-flat optical surface in the beam path will generally result in the two beams becoming unparallel, causing the distributed resonator to cease lasing. Such an optical component may cause each beam to be deflected differently, as is shown in FIG. 2, which illustrates the behavior of two parallel beams, one, marked Beam 1 passing through the optical center 21 of a lens 20, and one, marked Beam 2, passing through a point 23 displaced from the center. As is observed, after passage through the lens 20, the beams are no longer parallel. Since the two beams need to remain parallel for a distributed resonator to operate, as described in the aforementioned WO2007/036937 and WO/2009/008399, such an optical component cannot be used within a resonator having optical image inversion at its retroreflector(s) and having an opaque inversion point. Although it is possible to design certain optical elements to handle two parallel beams, such as a telescope lens arrangement, such a device may have a limited field of view and limited functionality, may require the separation between the beams to be fixed and may cause aberrations to both beams. This usually prevents the practical use of such telescope solutions. In U.S. Pat. No. 4,209,689 to G.J. Linford et al., for “Laser Secure Communications System”, there is described a distributed laser cavity for long range communication, with a telescope in the cavity close to the gain medium. This system deals with a beam which is very axially defined, and operates with as limited a field of view as possible, involving angles of propagation close to the axis. No mention is made of the longitudinal position of elements such as the gain medium, down the cavity length. It is believed that the telescope is used to expand the beam and hence to limit the beam divergence and field of view. In many other cases, there may not be need for a telescope, but rather for another optical element having a different function, such as a focusing lens, with the same problems arising therefrom because of the double beams.    c. An optical system designed for two beams needs to use components generally having diameters of at least twice the size as those of equivalent single beam systems, in order to accommodate the two beams and the distance between them. This would increase the cost of the system, and its overall width.    d. Usually, two simple retroreflectors are not enough to achieve lasing, since the beam typically needs to be focused in order to compensate for Rayleigh expansion. In the above referenced WO/2009/008399, this problem was solved by using a thermal focusing element. However such a solution suffers from increased complexity due to the need to initiate it.    e. Optical elements having optical power, such as those having at least one non flat surface, may be necessary in the beam's path to achieve other optical functions, such as focusing, to correct aberrations, to monitor the system's state, to change the field of view of the system or to work with different apertures to allow for better performance/price of the system. Since the two beams are essentially separated, it may also be difficult to block ghost beams, as an increased aperture is needed.    f. Since placing imaging optics inside the resonator is difficult, it is difficult to form an image of the position of a receiver. Such information may be potentially necessary to monitor a receiver or receivers connected to the transmitter.
An additional problem arises with the distributed laser systems shown in the above two referenced PCT publications, since the direction and position of the beams within the system are not known. It then becomes difficult to know where to place direction sensitive components in the beam's path, such as polarizers, waveplates, frequency doubling crystals, and the like. It also becomes difficult to know how to use position limited components, such as small detectors, gain media, and the like, since it is not known where to position such components laterally.
There therefore exists a need for a distributed laser cavity architecture which overcomes at least some of the above mentioned disadvantages of prior art systems and methods.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.